I can only vouch for my own experiences -- I've used the WIP threads to get feedback, and pretty much all of it was very positive, but then when I uploaded, the ratings told a different story, so I have ceased to post my works in progress, so that there wasn't a build up of expectation that would get dashed to pieces when the actual unveiling took place.
Often WIP threads and the like will attract a skinner's fans/followers [the author/nick of the OP attracts the interest] so subsequent critique can be somewhat 'rose-coloured'. When the skin is out in 'the wild' you will attract public users [who may never comment on the Forums at all] whose personal taste/opinion may be profoundly different.
There is a very real trend....I've seen it since day one...in the skin comments. Almost invariably the first comments will be "wow, cool skin...well done" [fan] but after the first few/half-dozen/page there will be "this effing thing is crap it doesn't even work properly". This is a perfect example of how a 'fan' can be less than useful/constructive. The question it raises is..."did these 'fans' even try the skin....how did it not not work for them?"
Often there will be 'trends' in ratings.... where a rating is applied not because of what...but because of who. This is typically either overly high...or overly low. [fan/non-fan]. Mostly that all balances out over time. If it doesn't then we look into it and check for trends [eg. drive-by]. Almost always one person will rate another's work with a spread of ratings....indicating merit, not person. If all of a person's ratings indicate foul play they get a 'please explain'....usually from me.
In reality, over more recent times, I have seen very little issues with skin ratings at all....so few that the issue isn't one at all...